1. Field of the Invention
The present invention relates generally to an inverter-drive controlling apparatus, and particularly concerns an inverter-drive controlling apparatus which is especially suitable for drive-controlling operation of an induction motor of a relatively small output power for industrial use such as for the compressor of an air conditioner, refrigerator, or the like.
There are several known types of control apparatus for inverters to drive a motor, such as of PAM, or PWM type. Among them, PWM of inequal width simulated sinusoidal wave is superior in power source utility, miniaturization and light weight of apparatus, low noise of electromagnetic wave, low mechanical noise, vibration, etc., and its use has become a major trend in recent years.
The PWM of simulated sinusoidal wave is, as shown in FIG. 3 and FIG. 5, a system to produce a PWM algorithm in a manner to simulate the sinusoidal wave with an integral value of pulse voltage fed to a motor winding.
Now, a prior art HALT system, which is a basis to make the present invention, is elucidated as the prior art, with reference to FIG. 1, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11 and FIG. 12. In FIG. 1, alternating current from a commercial power source E is rectified and smoothed by a reactifier-smoother 1, and the rectified and smoothed DC output from the rectifier-smoother 1 is given to an inverter. The output of the inverter 2 is fed to an electric motor 3, and an inverter drive controlling circuit 4 provides the inverter 2 with a controlling signal.
Next, one example of the general inverter system configured for an air conditioner is shown in FIG. 2.
In FIG. 2, numerals 1, 2, 3 and 4 designate the rectifier smoother 1, inverter 2, electric motor 3 and inverter drive controlling circuit 4 of FIG. 1, respectively. The inverter drive controlling circuit comprises a PWM algorithm generator 4a and a base current driver for supplying base currents to the bases of transistors Tr.sub.1, Tr.sub.2, Tr.sub.3, Tr.sub.4, Tr.sub.5, and Tr.sub.6 in the inverter 2 and a photo-coupler 4b which couples the PWM algorithm generator 4a and the base current driver 4c in insulated manner.
Signals generated by the PWM algorithm generator 4a are amplified and conveyed by the photo-coupler 4b to the base current driver 4c, and after current amplification the signals are routed to the inverter 2. In the inverter 2, the transistor pairs Tr.sub.1 and Tr.sub.2, Tr.sub.3 and Tr.sub.4, and Tr.sub.5 and Tr.sub.6 each constitute inverter switches, and one of each transistor in the pair is selectively turned on any time. Junction points between the transistor pairs are connected to three terminals U, V and W of the electric motor 3 of a compressor of an air conditioner.
FIG. 3 shows wave forms of signals to be applied to the bases of the transistors Tr.sub.1 through Tr.sub.6, and waveforms of voltages to be applied across the windings of electric motor 3. In FIG. 3, waveforms U, V and W correspond to the signals applied to the bases of the transistors Tr.sub.1, Tr.sub.3 and Tr.sub.5. U--V, V--W and W--U are waveforms of voltages applied to respective windings of the electric motor 3.
As shown in FIG. 3, the waveforms of the voltages are designed to simulate a sinusoidal wave when integrated, and a period of the pattern of this voltage determines the revolution of the electric motor 3.
Now, PWM algorithm is elucidated with reference to FIG. 4 which elucidates the concept of the carrier. A half period of the sinusoidal wave of FIG. 4 is equally divided by an integer N into N consecutive time periods. This integer N is called the carrier, and the small period T.sub.0 made by dividing the half period of the sinusoidal wave by the carrier N is called the carrier period. By issuing pulses in respective periods T.sub.0, with each having pulse widths proportional to a voltage which exists at that particular divided period T.sub.0 of the sinusoidal wave, the algorithm as shown by FIG. 3 is produced.
Nextly, the voltage value to be applied to the coils of the electric motor 3 is elucidated with reference to FIGS. 5(a) and (b). As shown by FIG. 5(a), it is provided that pulses of a predetermined voltage and having pulse widths corresponding to a sinusoidal wave having a value of an integral of the pulses are generated by means of an algorithm. When the pulse widths of the pulses are increased in a proportional way, the waveforms become as shown in FIG. 5(b). Namely, the value of the integral of the pulses increases. Accordingly, the amplitude of the sinusoidal wave can be controlled by changing the pulse widths.
Nextly, the relation between the pulse widths which defines the output voltage (amplitude of the sinusoidal wave) and HALT is elucidated with reference to FIG. 6(a) and FIG. 6(b). FIG. 6(a) shows a situation wherein the carrier period T.sub.0 (1) comprises plurally divided times: ome region of data, and a HALT region being defined as the remaining time in the carrier period T.sub.0 such as T.sub.0(i). By definition, in this HALT region, no voltage data is output. In this first case, shown in FIG. 6(a), the time period of the data region is much smaller in comparison with the carrier period T.sub.0 (1).
In a second case, as shown in FIG. 6(b), the carrier period T.sub.0 (1) is halved. Thus, in the time occupied by one T.sub.0 (1) period, two periods of duration T.sub.0 (2) are produced, and the time period of the data region is unchanged. Then, frequency f of the carrier becomes doubled (since carrier period T.sub.0 (2)=1/2.multidot.T.sub.0 (1)), and the output voltage is also doubled. The is because the relative pulse widths with respect to the carrier period T.sub.0 (2) are double the pulse widths with respect to the carrier period T.sub.0 (1).
A small time unit is defined by dividing the time period of the data region DATA of FIG. 6(a) and FIG. 6(b) by an integer K; this time unit is named the "data unit timer T.sub.2. Then, by fixing the data period or data unit timer T.sub.2 to a constant length and changing the carrier period T.sub.0, the frequency f is changed in an inverse proportion thereto, and output voltage is changed in proportion to the frequency. The HALT period, which is the period when no data is produced, also changes as in the example of FIGS. 6(a) and 6(b).
The above-mentioned frequency-output voltage relation is shown in FIG. 7.
Now, a further detailed description is made with respect to the data region, with reference to FIG. 8(a) and FIG. 8(b).
In these time charts, a sampled voltage during this period is represented by a number of the unit timers T.sub.2 periods. This number of periods corresponds to the value of the sampled voltage. Therefore, the voltage is represented by a logic pattern having K resolution.
When the carrier N and the integer K are selected as larger numbers, the waveform of the voltage to be applied to the electric motor more smoothly simulates a sinusoidal wave, with a real sinusoid being approached as N and K approach infinity.
As shown in FIG. 8(a) and FIG. 8(b), both cases have the same carrier period T.sub.0 (1), but the data unit timer T.sub.2 (1) of FIG. 8(a) is only half the length of time of the data unit timer T.sub.2 (2) of FIG. 8(b). Accordingly, the data region time length T.sub.2 (2).times.K of FIG. 8(b) is 2-times of the data unit timer T.sub.2 (1).times.K of FIG. 8(a), and HALT time of FIG. 8(b) accordingly becomes smaller than the HALT time of FIG. 8(a). In these cases, the output voltage of FIG. 8(b) is 2-times the output voltage of FIG. 8(a). Accordingly, the frequency-voltage graph of FIG. 9 plotted with the data unit timers T.sub.2 (1) and T.sub.2 (2) as parameters, becomes as shown in FIG. 9.
For a certain frequency, for instance, represented by a vertical line in FIG. 9, when voltage amplitude goes up, the parameter changes from T.sub.2 (1) to T.sub.2 (2) and so on, and the HALT region decreases; until at extremity, the HALT region vanishes. For a certain rectified and smoothed DC voltage from the rectifier smoother 1, a maximum voltage which can be impressed on the electrical motor 3 is determined by this condition. Accordingly, even though the frequency is increased further from that condition, the voltage to be impressed on the electric motor 3 does not change further. The above-mentioned situation is elucidated with reference to FIG. 10(a) and FIG. 10(b).
As shown in FIG. 10(a), a carrier period T.sub.0 (3) is equally divided by an integer number K thereby defining the data unit timer T.sub.2 (1)=1/K.multidot.T.sub.0 (3), without retaining the HALT region at all. That is, the relation T.sub.0 (3)=K.times.T.sub.2 (1) holds. Then, if the frequency f is raised so as to have a shorter carrier period T.sub.0 (4) shown in FIG. 10(b) than a previous carrier period T.sub.0 (3), the data unit timer T.sub.2 (3) is given by an equation T.sub.0 (4)=K.times.T.sub.2 (3). As the frequency change, ratios of data region period against carrier period T.sub.0 are kept constant, and accordingly the voltage obtained from the both cases are equal to each other as shown by FIG. 11.
Nextly, relation between the inverter output and load is elucidated. When the load is a resistive load, the inverter output is proportional to square of voltage. On the other hand, with respect to a compressor of an air for instance, amount of work is proportional to the exhaustion volume of refrigerant from cylinders of the compressor. Accordingly, the exhaustion volume is proportional to the number of revolutions of the electric motor. Accordingly, it is preferable that frequency f and the output voltage should have a predetermined proportional relationship.
However, an actual electric motor for the compressor shows effect of iron loss and copper loss, etc., and therefore, in a low frequency range it is necessary that its driving voltage should be increased in order to compensate these above-mentioned iron loss, copper loss, etc. That is, boost function is necessary.
In the prior art apparatus, the boosted curve was realized by adding corrections. This is accomplished by obtaining the carrier period T.sub.0 and data unit timer T.sub.2 by analog timer circuits. Then the carrier frequency is set by means of the carrier period T.sub.0, and the compensation is added to the unit timer T.sub.2 responding to the set value of the carrier period T.sub.0. The greatest advantage of the HALT system is that by only changing the carrier period T.sub.0 and data unit timer T.sub.2, the PWM algorithm can be obtained for any frequency regions by providing only one period of algorithm generation pattern.
The above-mentioned prior art apparatus has the advantage that, when the circuit used to produce the carrier period T.sub.0 and the data unit timer T.sub.2 are realized by analog timers, and minute variations of the timer value can be made by adjusting circuit components of the external circuit. Thus, the carrier period T.sub.0 and the data unit timer T.sub.2 can be adjusted independently of each other. But the prior art has a problem that when frequencies to be used are widely distributed and when close simulation to the sinusoidal wave is desired, there is a necessity that the carrier N and the integer K (number of data) should be altered for different conditions. That is, in a low frequency range where resolution of the sinusoidal wave becomes rough and simulation of the sinusoidal wave becomes difficult, it is necessary that the carrier N and the integer K must be selected large. And on the other hand, when the carrier N and the integer K are large in a range of high frequency of f, switching speed of the transistors Tr.sub.1 through Tr.sub.6 becomes a great problem. That is, due to limit of the switching speed of the transistors Tr.sub.1 through Tr.sub.6, the OFF-times of the transistors occupy a high ratio in the operation, and therefore the output voltage becomes low. Accordingly, the carrier N and the integer K must be limited to a reasonably small number.
In the prior art analog timer system, though PWM generation data itself relating to change of the carrier N and data number K can be made by an external data area such as ROM or the like, smooth switchings between two kinds or more analog timers is difficult in view of transiential phenomena. For instance, in case that such switching is made by changing the carrier period T.sub.0 and data unit timer T.sub.2 with even a small difference, target frequency or target voltage happens suddenly to change even at a short instance, and therefore the compressor may have an overcurrent or locking or at some instance, the power transistors will be damaged.